Implantable Cardioverter Defibrillators (ICDs) are implanted in patients susceptible to cardiac tachyarrhythmias including atrial and ventricular tachycardias and atrial and ventricular fibrillation. Such devices typically provide cardioversion or defibrillation by delivering low voltage pacing pulses or high voltage shocks to the patient's heart, typically about 500–800V. The ICD operates by using sensors to detect a fast heart rate or tachyarrhythmia, upon which a battery within the device housing is coupled via an inverter to a high voltage capacitor or capacitor pair to charge the capacitors. When the capacitor reaches a desired voltage, charging is stopped and the capacitors are discharged under control of a microprocessor to provide a therapeutic shock to the patient's heart.
The volume of the device is an important characteristic, with small size being desired for patient comfort. Given this, it is important that device life notbe sacrificed to achieve this, such as would occur if battery capacity were diminished to achieve a smaller size. Capacitors occupy a significant fraction of the device volume, and it is desirable to minimize the capacitor size without sacrificing the required charge storage capacity. Existing ICDs employ compact flat capacitors having a stack of highly etched anodes alternating with thin cathode foil layers (and separators between each cathode and anode.) A current capacitor design has a flat shape, with a peripheral profile shaped to fit efficiently within an ICD housing. An ICD contains two of these capacitors, overlaying each other in registration for a compact assembly. Each capacitor has a metal housing that is connected to the cathode layers, while the anode layers are isolated from the housing, and connected to a conductive “feed through” element that penetrates the housing through an insulative sleeve that preserves the leak-proof nature of the housing while providing a connection to the anodes.
While generally effective, the current capacitor design has some space inefficiencies that increase device volume without increasing capacitance. The housing design and feed through element occupy more space than would be desired. In addition, the use of two capacitors requires wiring connections between them and to circuitry that occupy volume in the device housing. Moreover, these wiring connections are vulnerable to damage during assembly, and can reduce production yields and require scrapping of components if damage occurs.
The use of two capacitors is necessitated in current designs by the required voltage for therapy, and by the technology used to form the capacitor components. To achieve the 830 voltage range required, two capacitors are connected in series, each capacitor employing costly advanced technology including specialized high-gain anode etching and dielectric formation that provides 415V per unit. While less-advanced technology can provide 250–375V capacitors at a much lower cost, this may necessitate a third unit in the assembly, increasing the volume consumed by the capacitors with another case, feed-through, and interconnection wires.